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PH-EP-2015-045

(Submitted on 03 Mar 2015)

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Abstract

A measurement of the cross-section for Z-boson production in the forward
region of pp collisions at 8TeV centre-of-mass energy is presented. The
measurement is based on a sample of $\rm Z\rightarrow e^+e^-$ decays
reconstructed using the LHCb detector, corresponding to an integrated
luminosity of 2.0fb$^{-1}$. The acceptance is defined by the requirements
$2.0<\eta<4.5$ and $p_{\rm T}>20$GeV for the pseudorapidities and transverse
momenta of the leptons. Their invariant mass is required to lie in the range
60--120GeV. The cross-section is determined to be $$ \sigma({\rm pp\to Z\to
e^+e^-})=93.81\pm0.41({\rm stat})\pm1.48({\rm syst})\pm1.14({\rm lumi})\;{\rm
pb}\,,$$ where the first uncertainty is statistical and the second reflects all
systematic effects apart from that arising from the luminosity, which is given
as the third uncertainty. Differential cross-sections are presented as
functions of the Z-boson rapidity and of the angular variable $\phi^*$, which
is related to the Z-boson transverse momentum.

Figures and captions

Overall detection efficiency, $\epsilon$,
determined from a combination of data and simulation as described in the text,
shown as a function of (left) $y_{ Z }$ and (right) $\phi^*$.

Comparison between data and simulation for the distribution of
$\min( p_{\rm T} ( e ^+ ), p_{\rm T} ( e ^- ))$, used in the assessment of uncertainties in $\epsilon_{\rm kin}$.
The data are shown as points with error bars, the background obtained from same-sign data is shown in red (dark shading),
to which the expectation from $ Z $ $\rightarrow$ $ e ^+ e ^-$ simulation is added in yellow (light shading).
The simulated distribution is normalised to the background-subtracted data.
The $\tau ^+\tau ^- $ background is also included (green), though barely visible.
The dashed line indicates the threshold applied in the event selection. The small plot at the top shows the pulls
( i.e. deviations divided by statistical uncertainties) between the data and the expectation.

Measured cross-section for $ Z \rightarrow e ^+ e ^- $ shown as the shaded band, with the inner
(orange) band indicating the statistical error and the outer (yellow) band the total uncertainty.
For comparison, the NNLO predictions of Fewz are shown using five different sets of PDFs.
The uncertainties on these predictions include the PDF uncertainties and the
variation of the factorisation and normalisation scales, as well as the errors arising from numerical integration.

(left) Differential cross-section ${\rm d}\sigma/{\rm d} y_{ Z }$ and (right)
normalised differential cross-section $(1/\sigma){\rm d}\sigma/{\rm d} y_{ Z }$
as a function of $y_{ Z }$.
The measured data are shown as the shaded bands, with the inner
(orange) bands indicating the statistical error and the outer (yellow) bands the total uncertainty.
For comparison, the NNLO predictions of Fewz using five
different sets of PDFs are shown on the left-hand plot.
The same data are compared with leading log calculations in the right-hand plot.
To aid clarity, small horizontal displacements are applied to some of the predictions.

(left) Normalised differential cross-section $(1/\sigma){\rm d}\sigma/{\rm d}\phi^*$
as a function of $\phi^*$.
The measured data are shown as the shaded bands, with the inner
(orange) bands indicating the statistical error and the outer (yellow) bands the total uncertainty.
For comparison, the predictions of the leading-log calculations described in the text are shown.
(right) The same data and predictions normalised to the measurement in data, so that
the measurements are shown as the shaded bands at unity.
To aid clarity, small horizontal displacements are applied to some of the predictions.

Tables and captions

Efficiencies and other factors used for the cross-section determination (see Eq. (1))
averaged over the experimental acceptance by integrating over $y_{ Z }$.
The fractional uncertainties on the overall factors are also given,
separated into components that are assumed to be correlated and uncorrelated
between bins of the differential distributions.

Differential cross-section for $ Z \rightarrow e ^+ e ^- $ as a function of $ Z $ -boson rapidity.
The first error is statistical, the second the uncorrelated experimental systematic,
the third the correlated experimental systematic and the last error
is the uncertainty in luminosity. The cross-sections are at the Born level,
i.e. before FSR.
The rightmost column gives values of the additional factor,
$f_{\rm FSR}$, by which the results should be multiplied in order to give the cross-sections after FSR.

Differential cross-section for $ Z \rightarrow e ^+ e ^- $ as a function of $\phi^*$.
The first error is statistical, the second the uncorrelated experimental systematic,
the third the correlated experimental systematic and the last error
is the uncertainty in luminosity. The cross-sections are at the Born level,
i.e. before FSR.
The rightmost column gives values of the additional factor,
$f_{\rm FSR}$, by which the results should be multiplied in order to give the cross-sections after FSR.